In a color cathode ray tube, a color selecting mechanism is provided in opposing relation to a color fluorescent screen to thereby cause an electron beam to land on predetermined fluorescent patterns.
In an ordinary color cathode ray tube, a shadow mask in which a single circular beam aperture, for example, is bored through a metal plate for a dot-shaped red, green and blue fluorescent triplet, for example, is provided in opposing relation to the color fluorescent screen as a color selecting mechanism. Such a shadow mask is supported to a frame by welding a circumferential portion of the metal plate molded as a dome shape by a press-treatment or the like. In this case, the shadow mask is supported to the frame without the application of tension so that, when a temperature of the shadow mask rises due to the electron beam scanned thereon, a so-called doming phenomenon which gives rise to a color misregistration is caused by the thermal expansion. To solve this problem, an Invar material having a low coefficient of thermal expansion is utilized as a mask material and the plate thickness thereof tends to increase in order to increase strength.
On the other hand, in the color cathode ray tube of the Trinitron type, three electron beams corresponding to red, green and blue colors are arranged on the horizontal plane and a color fluorescent screen is formed by arranging red, green and blue fluorescent stripes, each extending in the vertical direction, in a predetermined order in parallel. Also, an aperture grill, in which a large number of slits extended along the extending direction of the fluorescent stripes are formed, is disposed in an opposing relation to the fluorescent stripes as a color selecting mechanism.
In the ordinary aperture grill, as shown in FIG. 6 which is a schematic perspective view of an example of the ordinary aperture grill, a large number of slits 4 are bored through a metal plate 42 formed of a high purity iron thin plate having a thickness of 0.08 to 0.15 mm. This metal plate 42 is stretched over a frame 3. The frame 3 composed of a pair of opposing frame side members 3A, 3B an members 3C, 3D disposed across these frame side members 3A, 3B. The front end faces of the frame side members 3A, 3B are formed as curved surfaces forming the same cylindrical surface, and the metal plate 42 is stretched over these frame side members 3A and 3B.
When this metal plate 42 is stretched over and attached to the frame 3, the frame side members 3A and 3B of the frame 3 are drawn closer to each other by a turnbuckle. Then, under this condition, the metal plate 42 is secured at its edge portions corresponding to the respective ends of each slit 4 to the front end faces of the frame side members 3A and 3B by the welding-process. Thereafter, the external force applied to the frame 3 is released, whereby the band-shaped portions between the slits 4 on the metal plate 42 are extended in the extending direction of the slit 4 with a predetermined tension by a restitution force.
On the other hand, since the color cathode ray tube has recently become larger in size, the length of the band-shaped portion between the slits 4 of the metal plate 42 of the aperture grill 10 is increased so that, when an electron beam strikes the fluorescent screen, the band-shaped portion tends to vibrate due to vibration caused by sound, impulses or the like, which gives rise to problems such as occurrence of color misregistration or the like. Therefore, in order to suppress the vibration of the band-shaped portion, the thickness of the metal plate 42 is increased to increase rigidity, or the thickness of the material forming the frame 3 is increased to increase a resilient force which removes the above-mentioned distortion, thereby suppressing the vibration of the band-shaped portion.
The slits 4 are formed on the relatively thick metal plate 42 by etching both surfaces 42A and 42B of the metal plate 42 according to the photolithography technique. That is, as shown in FIG. 7A, a photoresist is coated on one surface 42A of the metal plate 42, is subjected to the pattern exposure, is developed, and is removed by the photolithography technique to form a predetermined stripe pattern through which openings 42AC are opened, an etching mask 11A being thus formed. Then, in a like manner, an etching mask 11B having openings 42BC, whose opening width is made large as compared with the width of the openings 42AC, is formed on the rear surface 42B in opposing relation to the pattern of the former etching mask 11A. Then, as shown in FIG. 7B, the first etching process is carried out, in which stripe-shaped grooves are formed on the two surfaces 42A and 42B by the etching process which uses an etchant such as FeCl.sub.3 (ferric chloride) or the like.
Then, as shown in FIG. 7C, a protecting film 12 such as a varnish or the like is coated on the stripe-shaped groove on the surface 42A side, and is used as an etching mask to carry out for the other surface 42B a relatively gentle etching with an etchant such as FeCl.sub.3 having a relatively low concentration until the protecting film 12 is exposed, as shown in FIG. 7D.
Thereafter, by removing the protecting film 12, the slit 4, whose cross section is substantially in an "8" letter shape, is formed as shown in FIG. 8. When the etching is carried out twice and the slit 4 is formed by the second etching whose etching rate is slow as compared with the case when the groove is formed by one etching-process, the etching time can be controlled with ease in reliable fashion so that an excess proceeding or the etching can be prevented. As a consequence, each etching depth can be formed with accuracy and therefore an effective width of the slit 4, i.e., a distance SW between the edges 7 produced by the etching process of the two surfaces, can be formed with excellent controllability and with high accuracy, even when the metal plate 42 is thick. However, this technique cannot avoid the problem that a workability is deteriorated as compared with the case where the groove is formed by one etching process.
When the edge 7 is formed as described above, a tapered portion 8 of a gentle curved shape is formed from the respective surfaces 42A, 42B to the edge 7. Accordingly, as shown in FIG. 9 which is a cross-sectional view illustrating that electron beams impinge upon a color fluorescent screen 5 when this aperture grill 10 is used, an incident electron beam Ei becomes incident on the color fluorescent screen 5 through the slit 4 to make the fluorescent dots of stripe shapes luminous. On the other hand, a reflected electron beam Er.sub.1 from the color fluorescent screen 5 due to the secondary emission is reflected on the surface of the aperture grill 10 and on the tapered portion 8 to cause scattered electron beams Es or a reflected electron beam Er.sub.2 to occur. As a result, the light emission of the color fluorescent screen 5 becomes inaccurate, which gives rise to the deterioration of color contrast and color purity. Further, when the slits 4 of the aperture grill 10 are formed through the thick metal plate 42 by one etching process, the surface area of the tapered portion 8 is increased more, which makes the problem of the deterioration of the color contrast and color purity more remarkable.
As described above, in the conventional color cathode ray tube of the Trinitron type, it is preferable that the aperture grill thereof uses the relatively thick metal plate 42. In this case, however, since the weight of the aperture grill 10 is increased because the resilient force must be increased in order to suppress the vibration as earlier noted, there is the problem that the total weight of the color cathode ray tube is unavoidably increased.
Furthermore, the width SW of the slit 4 which can be formed in the above-mentioned etching process is about 50% of a thickness t of the metal plate 42 due to the restrictions from an etching characteristic standpoint. For this reason, if the thickness of the metal plate 42 is increased, the width SW of the slit 4 is increased in proportion to the thickness t of the metal plate. There is then the problem that the slits cannot be densified, that is, the color cathode ray tube cannot be formed as a high definition color cathode ray tube.